The present invention relates to a method for applying a protective coating on a gas turbine engine blade having an internal cooling cavity. More particularly, the invention relates to the aluminide coating of the internal and external surfaces of such a gas turbine blade.
In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against the airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. (As used herein, the term turbine blade may refer to either a turbine blade or a turbine vane, which have similar appearance in pertinent portions.) The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine blades. In current engines, the turbine blades are made of nickel-based superalloys, and can operate at metal temperatures of up to about 1900–2100° F. (about 1038–1149° C.).
Turbine blades typically comprise cooling circuits that channel cooling air through the interior of the turbine airfoil to reduce temperatures encountered by the blade and improve part life. During operation of the jet engine, air is forced through the root portion of the blade, into the airfoil cooling chambers, and out openings at the external surface of the airfoil. The flow of the air removes heat from the interior of the airfoil and, in some cases, providing a boundary layer of cooler air at the surface of the airfoil. In at least some known blades, an abrupt transition extends between the root portion and the airfoil portion to increase the volume of cooling air entering the airfoil portion.
Gas turbine blades frequently have metallic surface coatings that are capable of resisting the oxidation, corrosion and sulfidation conditions generated during high temperature operation. Such coatings facilitate the airfoil withstanding thermal stresses that may be induced within the higher operating temperature areas of the blade. However, if the coating is applied at too great a thickness on regions of the blade operating at lower temperatures, such as the root and shank region, the combination of the increased coating thickness and the abrupt transition within the dovetail may cause cracking in the root portion as higher stresses are induced into the transition area of the dovetail. Over time, continued operation may lead to a premature failure of the blade within the engine.
The above coatings can be applied by depositing a vapor of one or more protective metals, for example aluminum or alloys of aluminum, on blade surfaces at high temperatures within a coating container or chamber commonly referred to as a “retort”. Generally, the blades to be coated are placed within the container, along with a source of the aluminide coating, typically in the form of metallic pellets retained in perforated baskets arranged in rows surrounding the blades. The coating container is then placed within a heater such as a furnace to generate an aluminide coating vapor. Generation of the coating vapor typically includes the use of halide “activators” such as fluorides, chlorides or bromides. The halide activator can be in the form of a gas that is introduced into the container to react with the source of the aluminide coating and form an aluminide-bearing gas, or it can be generated from a halide activator source within the container that forms a reactive halide gas upon heating.
The aluminide-bearing gas is typically transported or moved within the coating container by a nonoxidizing or inert carrier gas (e.g., hydrogen, nitrogen, helium or argon). In some vapor coating systems, this carrier gas is introduced through the bottom of the container and carries the aluminide-bearing gas upward to coat the blades. See, for example, U.S. Pat. No. 4,148,275 (Benden et al), issued Apr. 10, 1979; and U.S. Pat. No. 5,928,725 (Howard et al), issued Jul. 27, 1999. In other vapor coating systems, the carrier gas is introduced through the top of the coating container and diffuses throughout the container to carry the aluminide-bearing gas and coat the blades. See U.S. Pat. No. 6,039,810 (Mantkowski et al), issued Mar. 21, 2000.
It is desirable that a controlled, relatively uniform aluminide coating be applied to the external and internal surfaces of the turbine blades. It is also desirable that the aluminide coating applied to internal surfaces of the blades, particularly in the root and shank region, be relatively thin (for example, having a thickness of from about 0.0005 to about 0.0015 inches) (from about 12.7 to about 38.1 microns) so as not to cause premature cracking in the root portion of the blade.